Integrated devices or circuits are highly used in modern electrical circuit. High voltage integrated circuits (e.g. high-side gate drivers, high-side power switches, offline sensors, level shifters, etc.) have floating structures. Typically, these high voltage circuits are put on one tub, which also serves as a common reference node.
But sometimes sharing tub is not viable. If the high voltage circuits are not properly isolated from each other, undesirable parasitic device (e.g. minority injection) interaction may occur, which may lead to low efficiency parametric failure or even to worse destructive latch up.
In some cases, isolating the tub in high voltage floating circuits is required so that there is no unintentional interaction with other nearby circuits. But all these tubs need relatively going voltage wise together.